1. Field of the Invention
The present invention relates to a safety device for maintaining the safety of an apparatus by cutting off a current flow path in response to pressure buildup inside the apparatus. Particularly, the invention relates to a safety device for use in smaller size apparatuses such as rechargeable secondary batteries, typically lithium ion batteries, with an imperative need for size reduction. Further, the invention relates to a secondary battery using such a safety device.
2. Description of Related Art
With the spread of portable information systems such as portable phones, video cameras and notebook personal computers, high performance rechargeable secondary batteries such as lithium ion batteries have come into widespread use.
FIG. 9 is a schematic diagram illustrating the basic construction of a conventional lithium ion battery. The lithium ion battery has such a simple construction that a positive electrode 90 and a negative electrode 100 are disposed in an electrolytic solution 70 of an organic solvent with a separator 80 interposed therebetween. Migration of Li ions between the positive electrode 90 and the negative electrode 100 causes the battery to be charged and discharged. Lithium oxide is employed for the positive electrode 90, and a carbon compound is employed for the negative electrode 100.
The secondary battery offers high performance. However, when the battery is overcharged or short-circuited, a gas evolves due to decomposition of the electrolytic solution thereby to increase the inside pressure of the battery. In the worst case, this results in explosion of the battery. Therefore, batteries of this type are provided with various types of safety devices.
The safety devices are generally classified into two types, i.e., safety valve mechanisms and current flow interruption mechanisms. The safety valve mechanisms are designed such that a safety valve provided in a battery surface is opened by buildup of the battery inside pressure thereby to release the inside gas for prevention of further increase in the inside pressure. On the other hand, the current flow interruption mechanisms are adapted to cut off a conductive path by utilizing the buildup of the inside gas pressure or to cut off a conductive path in response to detection of a temperature rise caused by the pressure build-up inside the battery.
As a current flow interruption mechanism responsive to a temperature rise, a simple mechanism employing a positive temperature coefficient (PTC) element as a thermosensitive element has been put to practical use. However, there is a time lag between the buildup of the battery inside pressure and the temperature rise. Therefore, such a mechanism fails to ensure a stable operation, making it difficult to provide a highly reliable safety device.
A current flow interruption mechanism responsive to the buildup of the gas pressure inside a battery is disclosed, for example, in Japanese Unexamined Patent Publication (KOKAI) No. 9-55197 (1997). In accordance with this art, the mechanism includes a pressure-responsive displacement member adapted to be displaced by the increase in the battery inside pressure, and is designed so that a conductive path is cut off by the displacement of the pressure-responsive displacement member.
In this art, the conductive path, the pressure-responsive displacement member and a support member for supporting these components are required. Therefore, the mechanism has a complicated construction, presenting problems associated with the size reduction and cost reduction.
In recent years, the portable information systems have been remarkably downsized, and further size reduction thereof will inevitably be in demand. Therefore, the size reduction of the batteries to be incorporated in such portable information systems is imperative. Since a safety device including any of the aforesaid complicated mechanisms of the prior art has a relatively large size, it is very difficult to embody a smaller size battery with such a safety device.
It is a first object of the present invention to provide a safety device which has an easily downsizable construction.
It is a second object of the present invention to provide a safety device which can readily be incorporated in a battery.
It is a third object of the present invention to provide a highly reliable secondary battery provided with a safety device.
It is a fourth object of the present invention to provide an advantageous fabrication method for a highly reliable safety device.
The safety device according to the present invention includes a thin plate of a brittle material adapted to be ruptured by a difference between pressures exerted on opposite sides thereof, and a conductive path provided on the thin plate and adapted to be cut off by the rupture of the thin plate.
The thin plate may be provided integrally with a peripheral greater thickness portion.
One surface of the thin plate is preferably present in a reference pressure chamber. In this case, when a greater (or smaller) pressure is exerted on the other surface of the thin plate, the thin plate is ruptured, thereby cutting off the conductive path. Thus, the safety device is of a pressure-responsive type which is effectuated when the pressure is out of a predetermined allowable range.
More specifically, the safety device of the pressure-responsive type is preferably embodied such that the reference pressure chamber is defined by a partition wall partly constituted by the thin plate. In this case, the reference pressure chamber is preferably sealed.
The safety device may otherwise be constructed such that the reference pressure chamber is defined by a plate member having a cavity and a cover member which covers the cavity, and the cover member or a bottom of the cavity is comprised of the thin plate.
The safety device may otherwise be constructed such that the reference pressure chamber is partly defined by a spacer plate provided integrally with the thin plate on the one surface thereof and having a through-hole through which a portion of the one surface of the thin plate is exposed.
In this case, a juncture between the spacer plate and the thin plate preferably has an inner corner having a curvature radius of not greater than 0.1 mm. Thus, when the pressure difference between the opposite sides of the thin plate reaches a predetermined level, the rupture of the thin plate can instantaneously and assuredly be caused. Hence, the safety device is highly reliable.
Further, the reference pressure chamber is preferably sealed by a cover plate provided on a side of the spacer plate opposite from the thin plate as covering the spacer plate.
The reference pressure chamber preferably has a major- to minor-axis length ratio of 1.0 to 1.2 as viewed in plan. This ensures the rupture of the thin plate when the pressure difference between the opposite sides thereof reaches the predetermined level.
The brittle material may be selected from the group consisting of ceramic materials, glass materials and glass ceramic materials.
The brittle material preferably has a Young""s modulus of not lower than 60GPa and a flexural strength of 80 to 100 MPa.
The thin plate preferably has a thickness of 10 to 100 xcexcm. Thus, the thin plate is easy to handle in production thereof, and has stable rupture characteristics.
For more stable rupture characteristics of the thin plate, the thin plate preferably has a thickness of 25 to 75 xcexcm.
The secondary battery according to the present invention includes the aforesaid safety device which is interposed between an internal electrode and an external electrode in a battery casing.
Since the safety device has a small and simple construction, it is easy to incorporate the safety device in the battery. Thus, the secondary battery with the safety device is highly reliable with a smaller size.
The thin plate, if composed of a ceramic material, is virtually free from time-related changes, so that operational variations of the safety device can be suppressed for an extended period without provision of a special protective film on the thin plate.
The fabrication method for the safety device according to the present invention includes the steps of: forming a green sheet for a thin plate from a brittle material; forming a green sheet for a spacer plate having a hole which later serves as a reference pressure chamber; and combining the thin plate green sheet with the spacer plate green sheet into a laminate and integrally sintering the laminate.
In accordance with the present invention, the curvature radius of a corner of a juncture between the thin plate and the spacer plate can be reduced, and the safety device can be fabricated at lower costs.
In accordance with one embodiment of the present invention, the fabrication method further includes the step of forming a green sheet for a cover plate. In the combining and sintering step, the spacer plate green sheet is combined with the thin plate green sheet and the cover plate green sheet as being held therebetween and the three combined sheets are integrally sintered.
The fabrication method preferably further includes the steps of forming a through-hole in a portion of the cover plate which opposes to the reference pressure chamber and, after the sintering step, closing the through-hole.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.